Flexographic printing plates are widely used for printing of packaging materials ranging from corrugated carton boxes to cardboxes and to continuous web of plastic films. These plates are made from photosensitive material such as photocrosslinkable monomer-binder-initiator compositions and are sandwiched between polyester (e.g., Mylar® film) support sheets. Flexographic printing plates are characterized by their ability to crosslink or cure upon exposure to actinic radiation. Exposure and development of these plates typically consist of a uniform exposure of the back-side of the plate to a specified amount of actinic radiation. Next, an image-wise exposure of the front-side of the plate is made through an image-bearing art-work or a template, such as a photographic negative or transparency, e.g. silver halide films. The plate is exposed to actinic radiation, such as an ultraviolet (UV) or black light. The actinic radiation enters the photosensitive material through the clear areas of the transparency and is blocked from entering the black or opaque areas. The exposed material crosslinks and becomes insoluble to solvents used during image development. The unexposed, uncross-linked photopolymer areas under the opaque regions of the transparency remain soluble and are washed away with a suitable solvent leaving a relief image suitable for printing. Then the plate is dried. The printing plate can be further treated to remove surface tackiness. After all desired processing steps, the plate is mounted on a cylinder and used for printing.
For printing, surface quality and properties of a flexographic printing plate are important attributes. In practice prolonged exposure time is often necessary in order to hold or fully cure the smaller features such as the finer high-light dots, e.g. 1% or 2% dots, wherein % refers to the amount of area of paper covered with print ink, of high quality print images. The term “exposure latitude” describes the degree to which a photosensitive element can be over-exposed with only negligible image quality degradation. Photosensitive flexographic printing plates with larger exposure latitude are desirable as they are more tolerant to the actual exposure time used during front image-wise exposure, and are thus easier to use in practice.
One problem associated with flexographic printing plates is halation caused by the scattering of UV light within the non-image areas of the photopolymerizable medium. As nearly all heterogeneous photocrosslinkable compositions exhibit some degree of light scattering, prolonged image-wise exposure leads to a high level of background scattered actinic radiation, which is often sufficient to cause cross-linking or curing of polymer in regions not exposed to image-wise radiation. The overall effect of such unwanted cross-linking is the filling-in of fine negative, i.e., non-image, relief features and formation of broad shoulders, i.e., “halos”, around solid image areas. These lead to degradation in the print quality of flexographic printing plates and are linked to dot-gain which is the formation of a larger image dot size than intended.
Halation also degrades print-image quality by reducing plate exposure latitude. This problem is particularly pronounced in newly extruded plates. Exposure latitude can be further defined as the ability to simultaneously image low light throughput features, e.g., 1-2% dots, and high light throughput features, e.g., 4 mils reverse lines, onto a flexographic plate. To achieve wider exposure latitude and more stable properties, it is currently necessary to age-in newly extruded plates by storing them for 4-6 weeks. It is believed that during this age-in period, low levels of oxygen diffuse into the plate, and inhibit low level polymerization. Reducing or eliminating this “age-in” period would result in significant savings in inventory cost and shorten product cycle time.
Contributing to halation problems are capping layers which are sometimes added to flexographic printing plates to improve their ink transfer characteristics. Such capping layers may scatter actinic radiation very differently from the bulk of the printing plate, and due to its close proximity to the critical printing surface, minute differences in the refractive indices between the capping layer and the bulk can lead to unwanted photopolymerization of the critical printing surface. In addition, depending on the polymer composition, as much as 90% of scattered UV can originate from the top 25% of a photopolymerization layer.
Another problem with flexographic printing plates is contrast degradation caused by the graininess of image-bearing art-work. Most commercial image-bearing artworks are based on silver halides emulsion technology. As such, the graininess of silver halide particles often leads to a microscopically fuzzy image edge across the opaque and transmissive area of the art-work. This fuzzy image edge is partially transmissive for actinic radiation and thus can degrade image definition when the image is photochemically transferred from the artwork to the flexographic printing plate. The prolonged imagewise exposure needed for flexographic printing plates often leads to dot-gain. This problem is particularly pronounced in small (<1%) print dots, where the cross sectional area spanned by the fuzzy image edge is comparable to that of the print dots.
Another problem with flexographic printing plates is their thickness. Thinner plates are desired because they would make processing shorter, require less drying time, require less polymer and therefore would be cheaper. Deep relief would be needed to enhance imaging in such thin plates. It is generally more difficult to achieve high image quality in a thin plate construction. There are several reasons for this: 1) a thin plate requires more accurate control of the image relief depth, e.g. uncontrolled variation of relief depth across a printing plate can result in poor print image transfer; 2) the quality of relief image typically deteriorates, e.g., more “halo” develops, as the image relief depth decreases; and 3) the specular reflections of actinic radiation from the surfaces of a polyester support, and of a glass plate of an exposure unit can cause unwanted photopolymerization in the non-image areas of the photosensitive element.
A traditional approach to mitigate the effect of scattered actinic radiation is to increase the absorptivity of the polymeric medium such that scattered radiation is attenuated more efficiently. In one approach, passive UV absorbing dye is added to the flexographic printing plate formulation to increase absorptivity. While effective, this approach suffers from excessive loss in photospeed, as a proportional amount of radiation is siphoned off by the passive dye absorber. In another approach, higher photoinitiator concentration is used to raise the medium's absorptivity. While also effective, this approach too has shortcomings. The higher photoinitiator concentration renders the unexposed regions more sensitive to scattered actinic radiation, partly reversing the benefit gained from higher absorptivity.
Various passive non-bleaching dyes have been used as antihalation agents in color photography applications, and certain styryl and butadienyl dyes capable of being decolorized have also been proposed for use as antihalation agents in color photography (see U.S. Pat. No. 3,996,215). However, the azo-linkage of such dyes is too thermally unstable and readily undergoes trans→cis conversion at high temperature, i.e., thermal bleaching. This thermally induced absorptivity bleaching makes these azo-dyes unsuitable for melt-extruded flexographic printing plate application. In addition, photography applications involve the use of thin films where bulk light propagation effect and issues relating to volume through-cure are less important.
Certain styrylpyridinium compounds have been used as photobleachable dyes in water-soluble contrast enhancement layers (CEL) for photolithography for electronic devices (see Yonezawa, et al., Polym. Eng. Sci.(1989), 29 (14), pp. 898-901, and Japanese Pat. Appl. No. 63-121039 (1989)). Such systems typically consist of a layer of photoresist on top of a silicon wafer with the CEL covering the photoresist. The CEL contains a photobleachable dye, and the unbleached portion of the CEL acts in situ as a contact mask during exposure. Yonezawa et al. describe water-soluble contrast enhancement material composed of certain styrylpyridinium compounds and certain water-soluble polymer which showed thermal stability and photobleaching properties.
In U.S. Pat. No. 4,661,433 aryl nitrones are described as useful as photobleachable compounds in photolithography techniques for contrast enhancement. However, these dyes can be incompatible with unsaturated vinylic monomers or binders as they react readily with such monomers and binders at high temperature (about 120° C.) (see Hamer et al., Chemical Reviews, (1964), 64(4) pp. 474-498). In addition, the aryl-nitrone dyes are moisture sensitive leading to storage stability issues.
There is a need for a method of enhancing desirable flexographic printing plate properties, such as improved resistance to scattered light and wider exposure latitude, leading to higher image resolution in prints made by such plates. There is also a need for a method for providing images with a desired differentiation between image and non-image areas and a need to reduce or eliminate the age-in period for newly formed flexographic printing plates. There is further need for thinner flexographic printing plates.